The present invention relates generally to electrical cables. More particularly, the present invention relates to shielded electrical cables capable of preventing radiation from signals contained within, while also avoiding the creation of undesirable ground loops. In general, ground loop formation is an unintentional side effect of the process of cable shield connection to the terminating devices.
The use of shielded electrical cables for establishing suitable electromagnetic compatibility (EMC) margins in commingled communications, or other electronic equipment environments, is nearly ubiquitous. In such equipment settings, isolated single purpose connections are commonly utilized to bond conductive equipment shells to supporting frames and structures, and then through these, to earth or ground potential. This is primarily done to prevent hazardous voltage differences from developing between the exposed surfaces of the various entities so interconnected, and to improve signal integrity between equipment communicating over an electrical path. Nonetheless, easily measurable, and operationally problematic, voltage differences can result from any number of a variety of factors such as, for example, local fault currents, or external influences such as lightning, power system induction or faults, or even the ramifications of ambient magnetic disturbances created by solar storms.
In an effort to achieve suitable EMC margins, a shield conductor of a connecting cable is frequently connected at each end to an equipment shell. This practice, however, leads to the undesirable result of creating a complete electrical loop, which in the present context, is called a ground loop. Specifically, in this example, the ground loop consists of the preexisting equipment bonding mechanisms and the interconnecting cable shield. FIG. 1 provides a schematic representation of this condition. In FIG. 1, cable shield 10 is connected at opposite ends to a first equipment shell 12 and a second equipment shell 14, and thereby to area bonding network 16 to form a complete electrical loop or ground loop 18.
Often, the effects of the group loop are benign because there is little or no potential difference between cable ends, as a result of no external currents and a relatively small loop area as defined by the enclosing ground loop path. In other cases, however, a ground loop formed incidentally by the shield connections of the cable can create serious problems. For example, even though potential differences can be controlled by bonding system design to no more than a few volts, such a voltage can produce unintended cable shield currents of many amperes. This unintended current can, in turn, induce disturbances in other proximally located cables and, due to imperfections of shield construction, disturb the signals carried within the offending cable itself. Unreliable communication between interconnected equipment can result, and in rare instances, destructive levels can occur.
Therefore, because it is difficult to establish the immediate and future ramifications of incidental cabling ground loops, the routine creation of ground loops is to be avoided. Present practice is to avoid the creation of cable shielding ground loops by establishing a shield connection at only one end of the subject cable. By doing so, the continuous ground loop may be broken and the incidental and unwanted current flow in the shield interrupted.
This solution, however, is contrary to EMC best practices. In this regard, connecting the cable shielding at only one end of the cable gives rise to a number of other problems. These, in particular, include signal leakage radiation, and susceptibility to external radio frequency and other electromagnetic ambient conditions. To elaborate, shielding is used when it is desirable to prevent conducted signal leakage and resultant radiation from cabling. In a reciprocal manner, external electromagnetic fields are intended to induce currents on the cable shield, as opposed to the signal conductors contained within. Any discontinuity in the shield, such as intentionally disconnecting one end from the equipment shell at that end, to interrupt a ground loop path, for example, allows a voltage differential to develop across the discontinuity, with attendant undesirable coupling between external fields and the intentional signal currents.
To this date, the devices of the prior art have not been effective in addressing these and other problems. Current cabling designs alone cannot directly satisfy the contradictory goals of providing a continuous, and thus potentially effective, radio frequency (RF) shielding, and in the same instance, provide a discontinuous ground path, thus avoiding the formation of a ground loop. A well known, but rarely practiced solution because of induced mechanical complexities, and consequent cost penalties in equipment design, is to incorporate a discrete capacitor which is in series between the conductive equipment shell and each of the corresponding cable end shield connection means, in at least one of the devices to be interconnected, taken two at a time. For this purpose, a blocking capacitor typically in the order of 0.1 microfarads is selected, which must, along with its mounting means, possess very low stray inductive and resistive effects to avoid materially affecting shield RF performance as a result of its introduction.
A typical prior art cable 20 used for telecommunications equipment interconnections, which employs a metallized film shielding means is shown in cross-section in FIG. 2A. The metallized film used as the shield itself is shown in cross-section in FIG. 2B. Referring first to FIG. 2B, shield 28 is composed of a strip of nonconductive or insulating material 44 with a metallized layer 48 formed on one side. Referring next to FIG. 2A, shield 28 is helically or longitudinally wrapped around a plurality of conductors or signal leads 24. One edge of the metallized film shield, essentially parallel to the cable axis, is folded 40 so that when the shield material is formed around leads 24 and overlapped, the metallized surfaces so overlapped connect, forming an electrically continuous shield circumferentially. An uninsulated wire or drainwire 36, in turn, is wound in a widely spaced helix around shield 28 along its entire length in such a manner that it is in continuous contact with metallized outer layer 48. Drainwire 36 serves the purpose of mitigating the effects of the unavoidable shield seam, and when exposed at each cable end, provides a convenient means of connection to the cable shield. An insulating jacket 38 surrounds the shield 28 and the drainwire 36.
As in the case with this and any other form of shielded cable lacking isolation, connection of the shield to equipment enclosure at both ends in an environment where the enclosures are otherwise connected, in most instances by a grounding network, undesirably creates a ground loop.
Thus, prior art does not provide an economical or routine way to achieve simultaneously good cable RF shielding and avoidance of ground loop creation during interconnection of electronic equipment. Consequently, a need exists for a cabling mechanism which directly and economically addresses both performance goals at one time.
To address these and other needs of the prior art, it is an object of the present invention to provide a shielded cable capable of connecting, communications equipment in a manner that avoids the formation of undesirable ground loops while also avoiding signal radiation and unwanted external radio frequency and electromagnetic interference.
It is another object of the present invention to provide a shielded cable that incorporates a blocking capacitor within the shield construction itself.
It is yet another object of the present invention to provide a shielded cable that possesses the characteristics of capacitively coupled yet electrically isolated parallel shield surfaces.
It is yet another object of the present invention to provide a shielded cable which may be implemented utilizing, for example, simply constructed and applied shield material
To meet these and other objects, the present invention provides an electrical cable which includes one or more conductors; at least one shield encircling the at least one conductor, the shield extending along a length of the cable, each shield comprising at least one conductive layer separated electrically from at least another conductive layer by at least one nonconductive layer; and a plurality of connection mechanisms to the at least one conductive layer, each of the connection mechanisms being in substantially continuous contact with the at least one conductive layer of the at least one shield and being electrically separated from other conductive layers of other shields and from other connection mechanisms of said plurality of connection mechanisms, each of the connection mechanisms and each at least one conductive layer in contact therewith comprising one electrode of a plurality of electrodes electrically connectable at an end of the cable.
In one embodiment of the present invention, the electrodes of the electrical cable are electrically insulated from one another. Thus, the conductive layer of each of the shields is electrically separated from the conductive layers of other shields. Furthermore, each electrode may be connected to equipment at one end, with adjacent electrodes being connectable at an opposite end.
In another embodiment of the present invention, the cable includes two or more shields, with each shield and connection mechanism in contact therewith being connectable at one end of the cable and being positioned adjacent only shields and connection mechanisms connectable at an opposite end thereof.
In yet another embodiment of the present invention, each of the one or more shields may include one shield which has one nonconductive layer and two conductive layers formed thereon, with the nonconductive layer separating the two conductive layers. A related embodiment of the present invention includes shields comprised of a first tape and a second tape, each of the first tape and the second tape including a nonconductive layer and a conductive layer, the shield being arranged with the nonconductive layer of the first tape facing the nonconductive layer of the second tape. The second tape may also be oriented with the conductive surface of the second tape facing the nonconductive surface of the first tape, to provide increased inter shield capacitance per unit length and provide for one exposed surface of the shield assembly to be nonconductive, as desired.
In still another embodiment of the present invention, circumferential electrical continuity is facilitated by a first fold extending along a first end edge of the shield with the conductive layer facing outwardly and a second fold extending along a second end edge of the shield with another conductive layer facing outwardly, wherein the outwardly facing portion of the first end edge is in substantially continuous contact with a portion of the conductive layer spaced apart from the first end edge at a first predetermined position, thereby facilitating circumferential electrical continuity in the conductive layer, and wherein the outwardly facing portion of the second end edge is in substantially continuous contact with a portion of the another conductive layer spaced apart from the second end edge at a second predetermined position, thereby facilitating circumferential electrical continuity in the another conductive layer.
In contrast to the above embodiment, in another embodiment, the shields of the electrical cable include a first fold extending along a first end edge of the shield with the nonconductive layer facing outwardly, the outwardly facing portion of the first end edge separating the conductive layer from contact with other conductive layers of other shields.
In still other embodiments, the one or more conductors are grouped into two or more bundles of conductors, with each bundle of conductors being encircled by at least one shield of the one or more shields. Similarly, each bundle of the two or more bundles may just as easily be encircled by two or more shields, or encircled by one shield with all of the bundles in turn being encircled by another shield.
In yet other embodiments of the present invention, one or more of the conductive layers of the one or more shields includes a predetermined loss sufficient to control resonant effects introduced as a function of the exact cable length utilized. In contrast, in further embodiments, each nonconductive layer of the one or more shields includes a predetermined loss sufficient to control resonant length effects.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.